Characterization and Control of Mesopore Structural Heterogeneity for Low Thermal Maturity Shale: A Case Study of Yanchang Formation Shale, Ordos Basin

A series of integrated and repeatable methods were used to demonstrate the characterization and controls of the mesopore structural heterogeneity of low thermal maturity lacustrine shale. These methods include X-ray diffraction of bulk and clay minerals, total organic content and Rock-Eval analyses, Soxhlet extraction, low-pressure N 2 adsorption, and SEM-EDS. To research the pore structural heterogeneity in detail, we separate the pore-size fraction into four groups: <2 nm, 2–8 nm, 8–50 nm, and >50 nm. The laboratory data suggest that the specific surface areas and fractal dimensions are significantly higher for extracted samples than for unextracted samples. Pores of larger diameter display stronger structural heterogeneity. The residual bitumen, mainly contained in pores with diameters ranging from 2 to 8 nm, clogs these pores and stops N 2 from being adsorbed in these pores. Hence, residual bitumen decreases the quantity of adsorbed N 2 and affects the calculation of specific surface area and fractal dimensions. Basically speaking, residual bitumen does not decrease pore structural heterogeneity. Rock framework particles mainly affect the structural heterogeneity of pores with diameters from 8 to 50 nm. Kerogen and mixed-layer illite-smectite increases mesopore structural heterogeneity, while quartz and plagioclase decreases mesopore structural heterogeneity.